1. Field of the Invention
The present invention relates to a process for industrially advantageously preparing aminoacetaldehyde dialkyl acetals which are useful as intermediates for medicinals, agricultural chemicals, etc., and also as intermediates for modifiers for high molecular weight compounds such as starch.
2. Description of the Prior Art
Aminoacetaldehyde dialkyl acetals (hereinafter referred to as "aminoacetals") are compounds represented by the following formula: ##STR1## wherein R.sub.1 and R.sub.2 are each hydrogen or alkyl, and R.sub.3 is alkyl.
Typically, it is known to prepare these compounds by reacting a halogenoacetaldehyde dialkyl acetal with an amine, and it is generally reported that this reaction is conducted solely in a non-aqueous system (e.g. Org. Synth. III, 50 (1955); J. Amer. Chem. Soc., 71, 4002 (1949); Ger. Offen. 2,745,588, etc.).
However, the prior art method naturally requires strict selection of the solvent and in turn use of an expensive hazardous alkali substance such as metallic sodium or sodium alcoholate for neutralizing excess amine present in the reaction mixture during work up of the product. Also, in the prior art method, the amine starting material, which is normally a gas, must be used either in the form of a gas per se or as a non-aqueous organic solvent solution, both of which are relatively expensive, as compared to an aqueous solution, which would be very inexpensive. Thus, the known process is economically disadvantageous.
These disadvantages are said to be avoidable when the reaction is conducted in an aqueous system (e.g. Examined Japanese Patent Publication SHO 37-823), which led the present applicants initially to attempt to react methylamine, which is a gas, with chloroacetaldehyde dimethyl acetal in an aqueous system, then to neutralize the resulting reaction mixture, followed by salting out with an alkali for working up the product, and finally to extract the desired compound from the filtrate resulting from the salting out step.
When practiced, however, these latter method steps in an aqueous system were found to involve the separate problem of requiring a large amount of neutralizing alkali during work up and also a very large quantity of extraction solvent since the desired compound, namely, methylaminoacetaldehyde dialkyl acetal, had a very rich solubility in water, i.e. was very soluble in water and was only extractable therefrom with relative difficulty.
The above aqueous system reaction procedure also suffers from other problems. Since the hydrochloric acid, which results from the reaction as a by-product, forms an addition salt with the methylamine, used as starting material, the reaction undesirably consumes 2 moles of methylamine per mole of the other starting material, i.e. chloroacetaldehyde dimethyl acetal. Furthermore, this addition salt which is thereby present in the aqueous reaction system is objectionable in view of the pronounced tendancy thereof to cause corrosion of the reactor, thereby posing an equipment problem as well. Besides, it is impossible to conduct the reaction continuously because there is a need to cool the reaction system at one point in order to recover the excess methylamine remaining in the system after the reaction has been completed.